In a liquid chromatographic system, the LC column is located between an injector and an LC detector to separate one or more constituents of interest from the various interferences in a sample to permit detection of these constituents of interest by an LC detector.
Capillary LC is a micro-version of traditional liquid chromatography and its popularity has grown rapidly during the past decade. Capillary LC columns have extremely low solvent consumption and require ultra-low volumes of samples for analysis, and hence provide high efficiency separations. Analogous to traditional liquid chromatography, capillary LC consists of a micro-pump, a capillary column, a detector, and a data processing device. The capillary column is important to the system because it is where the separation process occurs.
A capillary LC column is manufactured by packing a capillary column with separation media, such as bonded silica particles, also referred to as packing material. As schematically illustrated in FIG. 1, a slurry is used in a packing process. The slurry includes a packing material mixed with a slurry solvent, and is housed in a reservoir adjacent the column. To pack the column, a packing solvent is pumped to the slurry reservoir to drive the slurry into the column, which allows the solvent to pass through but not the packing material, thereby packing the column with the separation media.
Different types of materials, such as fused silica, stainless steel, and polymer, have been used for the capillary column. Due to their unique features, fused silica capillaries are the most common for preparation of capillary LC columns. Fused silica capillary columns have inner diameters of less than 1 mm and, typically, less than 0.7 mm. They are strong and can withstand high packing pressure, e.g., at least 9000 psi. It is easy to control their column dimensions during manufacturing, and the columns do not deform during packing. Further, the wall of a fused silica capillary is smooth, which is very desirable for packing.
Although fused silica capillaries have some unsurpassed advantages, they do have certain limitations. The most significant limitation stems from the brittle and fragile nature of the glass-like material from which they are made. The frangible nature of a thin, fused silica tube makes packing, shipping, and handling difficult. A layer of polyimide is generally coated on the outside of the fused silica capillary for protection. However, if the polyimide layer has incurred even a small scratch during production or handling, it will lose its effect and the capillary can break with just a gentle touch.
To avoid damage to the packed capillary LC column, a shielding of stainless steel is sometimes provided for protection. Although the currently available steel shieldings do prevent the capillaries from breaking, they are rigid and thus require long connecting tubings to install the capillary column between the injector and the detector of an LC system. This generates unnecessary extra column dead volume which degrades separation efficiency. Moreover, a separate assembling process is required in addition to the packing process, which can add extra cost to capillary LC column production.
Another problem with fused silica capillary columns is the difficulty of achieving an exact length for the packed column. Errors of 1-2 mm in length are common.
In addition, a sleeve is needed to tighten and secure an end-fitting on the end of the capillary column. During the packing process, one end of the capillary is typically enclosed with an end-fitting assembly and the other end is connected to a slurry reservoir. A flexible sleeve is employed in the end-fitting assembly during packing because sufficient tightening is required to enclose the end for high pressure packing. The sleeve facilitates tightening and compensates for the size of the capillary, which is too narrow for the end-fitting. The packing pressure can force the end-fitting assembly open if there is insufficient tightening, while too much tightening can damage the capillary.
There is a need, therefore, for a device that can protect the capillary during packing and handling, and alleviate the other shortcomings of the fragile fused silica capillary.